The invention relates generally to fixed joints for holding several structural parts together and more particularly, it relates to a joint of wooden construction parts, especially for use in the construction of wooden furniture.
In manufacturing wooden furniture, there are necessary, in most cases, frames or other units in which wooden parts are joined at an angle and permanently connected such, as for example, joints in chairs, beds, cabinet doors and cabinet walls. Such joints are needed not only at the ends of corresponding furniture parts but also for example in doors and cabinet walls, for reinforcing the frame structure.
For this purpose different kinds of joints are employed.
In a known method a wooden part has concealed slitted hole or mortice into which is inserted a tenon provided on the wooden part to be joined. In another known method the parts to be connected are provided, instead of mortices and tenons, with one or more round bores into which matching pins are inserted. In still another method one wooden part in the range of the joint is provided with side-by-side arranged rectangular recesses so that a tenon results that extends across the full width of the wooden part. The matching wooden piece is made with a slit or recess corresponding to the width of the tenon and adapted for engaging with the elongated tenon. In more massive boards several tenons and slotted holes are employed.
In a further known method are also used the tenons and slotted holes in the aforementioned manner nonetheless the slotted holes and the tenons have a trapezoidal shape (dovetail joints) to attain an increased loading capacity.
All aforementioned kinds of joints have, however, several disadvantages. Especially in order to create a permanent joint the joint portions have to be glued and in prior-art joints the glued surfaces are relatively small and in most cases an accurate fitting of the joint parts cannot be achieved. In addition, to achieve a satisfactory adhesion of the glued parts it is necessary to press simultaneously in at least two directions the glued parts together. In joining the parts with the application of glue, there is a tendency that during the insertion of one part into the other the glue layer is partially pushed out from the glued surfaces. It accumulates either in front of the inserted tenon and consequently it may easily prevent a snug connection or it is discharged outwardly and looses its function.
Another substantial disadvantage of known joints is the fact that during the natural aging of the wood due to the residual moisture, progressive drying up or due to the exposure of the wooden parts to different temperature or air moisture conditions the danger is that the joint will become loose even after a short period of its use. Furthermore, the leverage of the part connected under an angle, for example in a chair, considerable loads are concentrated mostly on small surfaces of the tenon and this concentration of loads also frequently results in loosening of the joint already after a short period of usage.
To produce fitting joint parts having the required snug quality is in known kinds of joints very laborious and therefore costly. Except the joints consisting of round bores and tenons, the manufacture of slotted holes and tenons carries always the danger that too much wood is unintentionally removed and thus a waste product will result. Due to the notch effect the conventional joints are considerably susceptable to breakage.
As far as the construction is concerned, the known kinds of joints require the application of relatively massive wooden parts resulting in a high consumption of wood; if such joints are used for constructions of parts having small cross-sections, the load capacity of the furniture is impaired. If an additional structural part such as a back of a chair or a door panel of a cabinet is to be fastened to one or more parts united by the joint, it is necessary to take precautions as far as the shape of borings, slots or grooves is concerned or as regards the screw connections and the like.
Finally, in conventional joints it is always disturbing from the aesthetic appearance of furniture pieces when the slotted holes and tenons are visible from the outside.